1. Field of the Invention (Technical Field)
The present invention relates to apparatuses and methods for maintaining as constant as possible the melt rate during electro-slag remelting (ESR).
2. Background Art
In ESR melting, considerable effort is expended on control methodologies designed to keep the melt rate as constant as possible. The rationale behind these efforts is that, during the steady state portion of the melt, a constant melt rate leads to a constant solidification rate. Therefore, the ability to operate at a constant (and proper) solidification rate greatly increases the is likelihood that an ingot will be produced without solidification defects. In addition, a constant melt rate provides an environment where the reaction rates between the slag and molten metal remain constant as well. This results in a constant purification rate of the metal being processed and will eliminate variation in impurity levels.
Typical melt rate control schemes differentiate the data from a strain gauge based load cell system in order to obtain a melt rate. This method has numerous flaws. First, the mechanical system is prone to noise sources such as mechanical stiction, water pressure variations, electrical noise, and vibrations. Second, the melting current variations can cause changes in apparent weight, and third, buoyancy effects caused by the immersion of the electrode in the slag result in (measured) electrode weight becoming a function of electrode immersion depth.
All of these considerations limit the accuracy and response of prior art melt rate control schemes. As an example, a typical, commercially available ESR melt rate controller utilizes a twenty minute window to compute melt rate. This slow response time has been determined by equipment manufacturers to be necessary in order to filter out the induced noise. The interaction of the averaging time with the process induced weight variations results in an oscillation in the melt rate controller and effectively limits the accuracy of melt rate control. This limitation requires conservative processing parameters and, as a result, excess costs are incurred. Further, anomalies in the feed stock such as shrinkage cavities or "pipe" result in severe difficulties in the processing of segregation sensitive materials.
The present invention provides a much more accurate means of determining melt rate, namely by counting individual droplets, and thus an improved means of controlling the ESR process. Droplet counting has been possible in vacuum arc remelting (VAR) because so-called drip-shorts are easy to detect in the direct current (DC) based VAR systems. However, it has not heretofore been possible to count individual droplets in alternating current (AC) based ESR systems.